In a commonly-used electromagnetic fuel injection valve, an open valve state and a closed valve state are switched by presence or absence of energization, and period of the open valve state is adjusted by a period of an injection instruction pulse to adjust an injection amount of fuel. However, there are response delay times between a start of energization and opening of the valve and between an end of energization and closing of the valve; thus, the period of the injection instruction pulse is not necessarily equal to an actual injection period.
In addition, a valve element in the fuel injection valve does not move in a rectangular wave form like the instruction pulse but opens in accelerated motion, and when the valve is closed, the valve closes in accelerated motion. That is to say, the valve element moves in a quadratic curve shape with time.
Further, because the valve element cannot stop rapidly, the valve element collides with a component (valve seat or stopper) which defines a displacement of the valve element, thereby causing vibration (rebound) of the valve element. Due to this vibration, a relationship between a width (time) of the instruction pulse and the injection amount becomes non-linear instead of linear. Further, because the length of the period when this vibration continues depends also on accuracy of components constituting the fuel injection valve and other factors, individual variation of the fuel injection valve is a cause for the variation of the injection amount.
As described above, the response of the valve has instability due to the delay time and the vibration; thus, even when the width of the instruction pulse is made short, it is sometimes impossible to inject a sufficiently small injection amount of fuel. For this reason, there is a minimum value of the injection amount which the fuel injection valve can control, and this minimum value is referred to as a minimum injection amount.
In general, in order to make the minimum injection amount small, it is effective to increase spring force which biases the valve element in a valve closing direction so that the valve can be quickly closed after the instruction pulse ends.
However, in the case that a bias spring is set to provide a large load, when operating at a high fuel pressure, force acting in the valve closing direction increases, whereby it becomes difficult to open the valve. To address this issue, in a commonly-used fuel injection valve, the set load of the bias spring needs to be determined by trade-off between the minimum injection amount and an available fuel pressure.
As a conventional art addressing this issue, there is proposed an electromagnetic fuel injection valve which is configured such that a movable element driven by magnetic attractive force can move relative to a valve element which performs an opening/closing operation and such that a movable element is biased in the valve closing direction in a stationary state. In this electromagnetic fuel injection valve, in the stationary state, the movable element is in contact with a stopper provided on the valve element on an end face on a closed valve side; and an end face of the movable element on an open valve side is not in contact with the valve element but has a space therebetween. This space allows the movable element to free run without being in contact with the valve element when the fuel injection valve performs a valve opening operation, and after that, the end face of the movable element on the open valve side and the stopper of the valve element come into contact with each other to make the valve start to open (the valve element starts to move in the valve opening direction).
In a period of the above-described free-running of the movable element, the movable element is apart from the valve element; thus, the movable element can be accelerated without being influenced by a fuel pressure, whereby the valve opening operation can be easily performed even at a high fuel pressure.
As a result, the fuel injection valve having a structure in which the movable element can free run has an advantage that, even when the set load of the bias spring is made large, the valve opening operation can be easily formed at a high fuel pressure.
As the electromagnetic fuel injection valve described above, PTL 1 discloses an electromagnetic fuel injection valve in which the movable element is further divided into two pieces to be able to move relative to each other so that, also when the valve is closed, the movable element can free run, thereby accelerating the valve closing operation.
The electromagnetic fuel injection valve of PTL 1 is provided with the movable element which are made up of two pieces and loaded with a load by a first return spring and a valve closing body frictionally connected to the bigger one of the movable elements, and the first movable element part is loaded with a load in the closing direction by the first return spring, and the second movable element part is loaded with a load in the closing direction by a second return spring. As described above, by shortening not only the time necessary to open the valve but also the time necessary to close the valve, the minimum injection amount can be reduced.